Drum washers have a rotatable cylindrical drum, which is permeable to washing liquid and onto the case of which the pulp to be washed is fed constantly to form a mat. Washing liquid is conducted onto the top of the mat and it then passes through the mat and the case and enters the drum, from which it is removed through the drum end. The washed pulp is worked loose and removed from the periphery.
Drum washers are known wherein a mat is formed on the case and in the lower drum part with the aid of a special compression plate assembly. In this way the washing zone in the drum is lengthened. Such a washer is described e.g. in patent specification FI-C-55370.